linux-kernel - Re: [PATCH v2 05/14] futex: Add sys_futex

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Date:   Thu, 10 Aug 2023 14:01:20 -0300
From:   André Almeida <andrealmeid@...lia.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        dvhart@...radead.org, dave@...olabs.net, tglx@...utronix.de,
        axboe@...nel.dk, Andrew Morton <akpm@...ux-foundation.org>,
        urezki@...il.com, hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        malteskarupke@....de, Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v2 05/14] futex: Add sys_futex_wake()



Em 10/08/2023 09:13, Peter Zijlstra escreveu:
> On Wed, Aug 09, 2023 at 07:25:19PM -0300, André Almeida wrote:
>> Hi Peter,
>>
>> Em 07/08/2023 09:18, Peter Zijlstra escreveu:
>>> To complement sys_futex_waitv() add sys_futex_wake(). This syscall
>>> implements what was previously known as FUTEX_WAKE_BITSET except it
>>> uses 'unsigned long' for the bitmask and takes FUTEX2 flags.
>>>
>>> The 'unsigned long' allows FUTEX2_SIZE_U64 on 64bit platforms.
>>>
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>>> Acked-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>>> ---
>>
>> [...]
>>
>>> +/*
>>> + * sys_futex_wake - Wake a number of futexes
>>> + * @uaddr:	Address of the futex(es) to wake
>>> + * @mask:	bitmask
>>> + * @nr:		Number of the futexes to wake
>>> + * @flags:	FUTEX2 flags
>>> + *
>>> + * Identical to the traditional FUTEX_WAKE_BITSET op, except it is part of the
>>> + * futex2 family of calls.
>>> + */
>>> +
>>> +SYSCALL_DEFINE4(futex_wake,
>>> +		void __user *, uaddr,
>>> +		unsigned long, mask,
>>> +		int, nr,
>>> +		unsigned int, flags)
>>> +{
>>
>> Do you think we could have a
>>
>> 	if (!nr)
>> 		return 0;
>>
>> here? Otherwise, calling futex_wake(&f, 0, flags) will wake 1 futex (if
>> available), which is a strange undocumented behavior in my opinion.
> 
> Oh 'cute' that.. yeah, but how about I put it ...
> 
>>> +	if (flags & ~FUTEX2_VALID_MASK)
>>> +		return -EINVAL;
>>> +
>>> +	flags = futex2_to_flags(flags);
>>> +	if (!futex_flags_valid(flags))
>>> +		return -EINVAL;
>>> +
>>> +	if (!futex_validate_input(flags, mask))
>>> +		return -EINVAL;
> 
> here, because otherwise we get:
> 
> 	sys_futex_wake(&f, 0xFFFF, 0, FUTEX2_SIZE_U8)
> 
> to return 0, even though that is 'obviously' nonsensical and should
> return -EINVAL. Or even garbage flags would be 'accepted'.
> 
> (because 0xFFFF is larger than U8 can accomodate)
> 

That make sense to me, but we would also want to validate the value of 
f, if it's NULL or something strange to return -EINVAL... but this 
happens only inside get_futex_key()...

To make this right, I think we would need to move this verification to 
the syscall validation part:

	if (unlikely((address % sizeof(u32)) != 0))
		return -EINVAL;

	if (unlikely(!access_ok(uaddr, sizeof(u32))))
		return -EFAULT;

And have u32 replaced with the proper size being used.

>>> +
>>> +	return futex_wake(uaddr, flags, nr, mask);
>>> +}